1. Field of the Invention
The present invention relates to antiozonants. More particularly, the present invention relates to mixtures of certain N,N′-disubstituted p-phenylenediamines that are useful as antiozonants for rubber.
2. Description of Related Art
Unsaturated rubber is subject to attack and degradation by ambient ozone, which attacks and breaks the unsaturated carbon-carbon linkages of the polymer chain. This results in the formation of cracks that eventually cause the rubber article to fail. Rubber is also degraded by the action of heat and ambient oxygen, which causes weakening and failure of the rubber. Also, rubber articles that are used in dynamic applications, where they are repeatedly flexed, can fail prematurely owing to cracking brought about by the repeated flexing.
Materials that are particularly useful for the protection of rubber are those that combine the activity of antiozonants, antioxidants, and antifatigue agents, and that provide a high level of activity in each of these areas. It has been known for some time that N,N′-disubstituted para-phenylenediamines may be used to protect unsaturated rubbery polymers from the deleterious effects of ozone and oxygen. Examples of such antiozonants in common use include N-(1,3-dimethylbutyl)-N′-phenyl p-phenylenediamine; N-isopropyl-N′-phenyl-p-phenylenediamine; N-(1,4-dimethylpentyl)-N′-phenyl-p-phenylenediamine; N-(1-methyl-heptyl)-N′-phenyl-p-phenylenediamine; N-cyclohexyl-N′-phenyl-p-phenylenediamine; mixed diaryl-p-phenylenediamines; N,N′-diphenyl-p-phenylenediamine; N,N′-di-beta-naphthyl-p-phenylenediamine; N,N′-bis(1,4-dimethylpentyl)-p-phenylenediamine; N,N′-bis(1-ethyl-3-methylpentyl)-p-phenylenediamine; N,N′-bis(1-methylheptyl)-p-phenylenediamine; and N-phenyl-N′-p-toluenesulfonyl-p-phenylenediamine.
Additionally, certain para-phenylenediamines can protect the rubber from cracking and failure to due to repeated flexing. These materials can also possess certain undesirable properties, such as a tendency to reduce the scorch safety of the rubber, or to produce an unsightly bloom on the surface of the rubber article. The most useful materials will be those that contribute minimally to these undesirable properties while maintaining a high level of antidegradant activity. Materials such as N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine, (6-PPD), and N,N′-ditolyl-p-phenylenediamine are known to provide good protection to rubber. These materials are widely used in the rubber industry, but they suffer from the problems described above. In particular, 6-PPD will form a reddish brown bloom on the surface of rubber articles in which it is used. N,N′-ditolyl-p-phenylenediamine has only limited solubility in rubber that limits its useful level to about two parts per hundred parts of rubber.
A number of blends of p-phenylenediamines have been proposed with the objective of improving antiozonant activity. These are described in U.S. Pat. Nos. 3,530,076; 3,645,966; and 3,663,505.
U.S. Pat. Nos. 3,530,076 and 3,645,966 disclose synergistic antiozonant mixtures of from about 25% to about 75% by weight of N-isopropyl-N′-phenyl-p-phenylenediamine and correspondingly from about 75% to about 25% by weight of N,N′-di-sec-alkyl-p-phenylenediamine containing from 3 to 10 carbon atoms in each alkyl, and the use thereof in rubber.
U.S. Pat. No. 3,663,505 discloses antiozonant compositions for use in rubber comprising a mixture of N,N′-di-alkyl-p-phenylenediamine in which each alkyl contains 6 to 10 carbon atoms, N-phenyl-N′-alkyl-p-phenylenediamine in which the alkyl contains 3 to 6 carbon atoms, and N-phenyl-N′-alkyl-p-phenylenediamine in which the alkyl contains 7 to 10 carbon atoms.
The blends described in these patents all contain various amounts of N,N′-dialkyl-p-phenylenediamines. Although the compounds are protected, this blend is less desirable since N,N′-dialkyl p-phenylenediamines are known to cause a decrease in the scorch time of rubber compounds in which they are used. They also tend to be less persistent, since they are more easily oxidized by air. (See Kuczkowski, J., Oxidation Inhibition In Organic Materials, Vol. 1, Ch. 8, Part VII., Ed. Pospisil, J., & Klemchuk, P. P., CRC Press, 1990, p. 276-281.)
Blends are also used where the particular properties of two or more paraphenylenediamines are desired in a single product.
European Patent Application No. 084,527 discloses a process for preparing a mixture of two or more different N,N′-disubstituted paraphenylenediamines by the reductive alkylation of a nitrogen-containing compound selected from 4-nitrodiphenylamine, 4-aminodiphenylamine, paranitroaniline and phenylenediamine with two or more ketones wherein the ketones are charged sequentially and each ketone is reacted essentially to completion before charging the next.
U.S. Pat. Nos. 6,706,216 and 6,706,217 disclose liquid antiozonant mixtures obtained from the process comprising simultaneously reacting at least one unsubstituted and/or substituted paraphenylenediamine compound with a carbonyl compound mixture comprising acetone and at least one other carbonyl compound selected from the group consisting of ketones containing from 4 to about 12 carbon atoms, aldehydes containing from 1 to about 12 carbon atoms and mixtures thereof in the presence of a reductive alkylation catalyst. Rubber compositions containing the liquid antiozonant mixtures are also disclosed.
Additionally, N,N′-disubstituted p-phenylenediamines have been found to be useful as antioxidants for hydrocarbons other than rubber. Certain N,N′-disubstituted p-phenylene-diamines are known to function as antioxidants for lubricating oils. U.S. Pat. No. 5,232,614 discloses lubricating oil compositions for use in environments where iron catalyzed oxidation can take place, which contain N-alkyl-N′-phenyl-p-phenylenediamines in which the alkyl groups contain up to 20 carbon atoms.
The disclosures of the above are incorporated herein by reference in their entirety.